Hydrotreating-hydrocracking process for manufacturing gasoline range hydrocarbons

ABSTRACT

A process for converting sulfur and nitrogen containing hydrocarbons boiling in the 400*-1,000* F. range into gasoline range hydrocarbons boiling below about 400* F. which comprises hydrotreating the sulfur and nitrogen containing hydrocarbons to yield a liquid hydrocarbon product containing less than 1 ppm nitrogen, and hydrocracking such hydrotreated hydrocarbon to yield gasoline boiling range product. The hydrotreating reaction is improved by employing pressures in the range of from about 1,200-1,500 psig and hydrogen to hydrocarbon ratios in the range of 10,000-20,000 SCF/B. By operating the hydrocracking reaction at a higher pressure than the hydrotreating reaction only one hydrogen circulation system is required wherein a hydrogen recycle gas is from the hydrocracking reaction effluent is charged to the hydrotreating reaction, wherein a gas stream comprising hydrogen, hydrogen sulfide and ammonia separated from the hydrotreating reaction effluent is treated to remove hydrogen sulfide and ammonia, wherein such treated gas is mixed with makeup hydrogen, compressed and recycled to the hydrocracking reaction.

United States Patent Wrench et al.

1 Dec. 18, 1973 HYDROTREATING-HYDROCRACKING PROCESS FOR MANUFACTURINGGASOLINE RANGE HYDROCARBONS [75] Inventors: Richard E. Wrench, Houston;

Benjamin F. Smith, Jr., Groves, both of Tex.

[73] Assignee: Texaco Inc., New York, NY.

[22] Filed: Dec. 29, 1971 [21] Appl. No.: 213,643

[52] US. Cl. 208/89, 208/59 [51] Int. Cl Cl0g 23/00 [58] Field of Search208/89, 209, 59

[56] References Cited UNITED STATES PATENTS 3,256,178 6/1966 Hass et al.208/89 3,549,515 12/1970 Brainard et al.

3,644,197 2/1972 Kelley et al 208/89 Primary ExaminerDelbert E. GantzAssistant Examiner-S. Berger Att0rney-Thomas H. Whaley 5 7 ABSTRACT Aprocess for converting sulfur and nitrogen containing hydrocarbonsboiling in the 400l,000 F. range into gasoline range hydrocarbonsboiling below about 400 F. which comprises hydrotreating the sulfur andnitrogen containing hydrocarbons to yield a liquid hydrocarbon productcontaining less than 1 ppm nitrogen, and hydrocracking such hydrotreatedhydrocarbon to yield gasoline boiling range product. The hydrotreatingreaction is improved by employing pressures in the range of from about1,200-1,500 psig and hydrogen to hydrocarbon ratios in the range of10,000-20,000 SCF/B. By operating the hydrocracking reaction at a higherpressure than the hydrotreating reaction only one hydrogen circulationsystem is required wherein a hydrogen recycle gas is from thehydrocracking reaction effluent is charged to the hydrotreatingreaction, wherein a gas stream comprising hydrogen, hydrogen sulfide andammonia separated from the hydrotreating reaction effluent is treated toremove hydrogen sulfide and ammonia, wherein such treated gas is mixedwith make-up hydrogen, compressed and recycled to the hydrocrackingreaction.

PATENTED DEC 18 I975 1 llllYDROTREA'IING-HYDROCRACKING PROCESS FORMANUFACTURING GASOLINE RANGE I-IYDROCARBONS BACKGROUND OF THEINVENTION 1. Field of the Invention The present invention relates to themanufacture of gasoline by the process of hydrotreating a sulfur andnitrogen containing petroleum fraction boiling in the range of fromabout 400 F. to about l,000 F. and subsequently hydrocracking liquideffluent from the hydrotreating step. In the hydrotreating step, thesulfur and nitrogen compounds contained in the petroleum fraction areconverted into hydrogen sulfide and ammonia respectively. The hydrogensulfide and ammonia are separated from the liquid hydrocarbon portion ofthe hydrotreating reaction effluent and the liquid hydrocarbon, reducedin sulfur and nitrogen content, is charged to the hydrocrackingreaction. In the hydrocracking reaction, hydrocarbons boiling aboveabout 400 F. are hydrocracked into hydrocarbons boiling below about 400F. A portion of the high boiling hydrocarbons are unconverted in thehydrocracking reaction. The effluent from the hydrocracking reaction isseparated into a gasoline product fraction and a fraction boiling above400 F. The fraction boiling above 400 F. recovered from thehydrocracking reaction is recycled for further conversion to thehydrocracking reaction.

In the hydrotreating reaction, molecular hydrogen is reacted with sulfurand nitrogen compounds to form hydrogen sulfide and ammonia at anelevated temperature and super-atmospheric pressure, in the presence ofa hydrotreating catalyst. In the hydrocracking reaction, petroleumhydrocarbons boiling above about 400 F. are converted into hydrocarbonsboiling below about 400 F. with molecular hydrogen, at an elevatedtemperature, a superatmospheric pressure, and in the presence of ahydrocracking catalyst.

2. Prior Art It is well known to convert hydrocarbons boiling in therange of 400 l,000 F. into a gasoline fraction boiling in the range ofabout 100 400 F. by hydrocracking such high boiling hydrocarbonfractions. I-Iydrocracking operating conditions include temperatures inthe range of from 400 800 F., pressures of from 500 to 3,000 psig,liquid hourly space velocities (LI-ISV) of 0.5-l5 volumes of oil perhour per volume of catalyst and hydrogen to hydrocarbon ratios of fromabout 3,000 to 20,000 standard cubic feet of hydrogen per barrel of oil(SCF/B). Hydrocracking reactions are performed in the presence of ahydrocracking catalyst comprising a hydrogenation component and acracking component. Hydrocracking catalyst employed may comprise acombination of a refractory cracking base with a suitable hydrogenationcomponent. Suitable cracking bases include, for example, mixtures of twoor more refractory oxides such as silica-alumina, silicamagnesia,silica-zirconia, alumina-boria, silica-titania, silica-zirconia-titania,acid treated clay, and the like.

- Additionally, cracking bases comprising partially dehydrated zeoliticcrystalline molecular sieves of the X or Y crystal types, havingrelatively uniform pore diameters of about 8 to 14 angstroms,andcomprising silica,

a relatively high siOlAL O ratio, e.g.,'between aboutv 2.5 and 6.0. Themost active forms of the molecular sieves are those wherein theexchangeable zeolitic cations are hydrogen and/or a divalent metal suchas magnesium, calcium or zinc.

The hydrogenation components of the hydrocracking catalyst arecompounded with the foregoing cracking bases by methods such asimpregnation. The hydrogenation component consists of from about 0.5percent to about 25 percent of a group VIB or group VIII metal compoundsuch as an oxide or sulfide of chromium, tungsten, cobalt and nickel,the corresponding free metals, or any combination thereof.Alternatively, smaller portions, between about 0.5 and 2 percent, of themetals platinum, palladium, rhodium or irridium or mixtures thereof maybe employed as the hydrogenation components. The oxides and sulfides ofother transition metals may also be used as hydrogenation catalysts butto less advantage than the foregoing.

Petroleum fractions employed for conversion into gasoline byhydrocracking commonly contain sulfur and/or nitrogen compounds. Suchsulfur and nitrogen compounds, when contacted with a hydrocrackingcatalyst under hydrocracking conditions, have a deleterious effect uponsuch catalysts. It is known to remove such sulfur and nitrogen compoundsby hydrotreating prior to hydrocracking a petroleum fraction. In such ahydrotreating reaction, sulfur and nitrogen contained in the chargepetroleum fraction are converted into hydrogen sulfide and ammonia.Effluent from the hydrotreating reaction is treated to remove thehydrogen sultide and ammonia and a treated hydrocarbon effluent,deficient in sulfur and nitrogen compounds, is then charged to thehydrocracking reaction. By removing such sulfur and nitrogen compoundsfrom the charge petroleum fraction, the hydrocracking reaction may beoperated at a lower temperature to obtain a desired degree of conversioninto gasoline boiling range hydrocarbons. By removing sulfur andnitrogen compounds which poison the catalyst, lower reactiontemperatures may be used which serve to extend the useful life of ahydrocracking catalyst.

The hydrotreating reaction for the conversion of sulfur and nitrogencompounds is performed under operating conditions including temperaturesin the range of about 600 800 F., pressures of 500 3,000 psig, LI-ISV of0.5-l0 volumes of oil per hour per volume of catalyst (Vo/I-Ir/Vc), andhydrogen to hydrocarbon ratios of 1,000 20,000 (SCF/B). Thehydrotreating reaction is performed in the presence of a suitablehydrotreating catalyst. Suitable hydrotreating catalyst include forexample, mixtures of the oxides and/or sulfides of the group VI B and/orgroup VIII metals. Preferably, the hydrotreating catalysts are supportedupon a refractory metal oxide carrier such as alumina, silica, titania,and the like. In the hydrotreating reaction, it is preferred to reducethe nitrogen content and the sulfur content of the charge petroleumfraction to low levels in the range of a few parts per million tominimize the effect of such compounds upon the hydrocracking catalystsin the subsequent hydrocracking reaction.

One process scheme known to the prior art comprises hydrotreating asulfur and nitrogen containing petroleum fration with a large excess ofhydrogen in the presenceof a hydrotreating catalyst; passing the totalhydrotreating effluent including hydrogen, hydrogen sulfide, ammonia,and hydrocarbons into a hydrocracking zone'wherein high boilinghydrocarbons are converted into gasoline boiling range hydrocarbons. Insuch a process, the hydrogen sulfide and ammonia present in thehydrotreating reaction effluent have an adverse effect upon thehydrocracking catalyst. Such hydrogen sulfide and ammonia poison manyhydrocracking catalyst extremely rapidly, making them unsuitable in sucha process. Other hydrocracking catalysts, which can withstand thepresence of hydrogensulfide and ammonia, are nevertheless affected bythe presence of such compounds and increased operating severities arerequired to obtain a desired degree of conversion. Increased operatingseverities in the hydrocracking reaction reduce the period for which ahydrocracking catalyst may be employed before it must be regenerated andincrease the rate of production of low molecular weight hydrocarbons inthe C -C range which are unsuitable for use in gasoline product.

In another process scheme, hydrogen sulfide and ammonia are separatedfrom a hydrotreated petroleum fraction prior to charging such fractionto a hydrocracking reaction. Effluent from a hydrotreating reaction isseparated into a gas fraction comprising hydrogen, hydrogen-sulfide andammonia and a liquid fraction substantially free of hydrogen sulfide andammonia. At least a portion of the gas fraction is returned to thehydrotreating reaction as recycle hydrogen and the hydrotreated liquidhydrocarbon fraction is charged to a hydrocracking reaction. Thisprocess scheme prevents hydrogen sulfide and ammonia from contacting thehydrocracking catalyst. Consequently, less severe operating conditionsmay be employed in the hydrocracking reaction. This scheme requiresseparate hydrogen recycle systems for both the hydrotreating reactionand the hydrocracking reaction. The two separate hydrogen recyclesystems increase the installation costs and operating expenses for sucha process scheme.

The effective life of a hydrocracking catalyst to convert a high boilingpetroleum fraction into gasoline range hydrocarbon is increased as thenitrogen content of such petroleum fraction is decreased. The prior artteaches that it is desirable to hydrotreat a petroleum fraction toobtain a hydrocracking reaction charge stock containing less than 25 ppmnitrogen compounds in order to prevent rapid poisoning of thehydrocracking catalyst. As the hydrocracking catalyst is deactivated,the operating temperature must be increased at a constant charge rate toobtain the desired conversion of hydrocracking charge hydrocarbons intogasoline range hydrocarbons. An increased operating temperatureincreases the coke deposition rate upon hydrocracking catalyst and theperiod of useful catalyst life is shortened. In the prior art methods,it is necessary to increase the hydrotreating reaction severity withincreased catalyst age in order to maintain the nitrogen content of ahydrocracking reaction charge stock within the desired range. Commonly,either the LHSV of the petroleum fraction is decreased or the operatingtemperature is increased in the hydrotreating reaction. As it is noteconomical to operate a hydrotreating reaction at a very low LHSV, belowabout 0.5 Vo/I'Ir/Vc, generally the operating temperature is increasedto improve the conversion of nitrogen compounds in the petroleumfraction being hydrotreated. Such increase in temperature shortens theperiod of effective catalyst life for the hydrotreating catalyst.

SUMMARY OF THE INVENTION We have observed that the period of effectivecatalyst life for a hydrocracking catalyst is substantially increased ifthe nitrogen content of a petroleum fraction charged to thehydrocracking reaction is maintained below 1 ppm. For petroleumfractions with such low nitrogen content, the hydrocracking reactiontemperature may be maintained at a relatively low temperature for anextended time period. The relatively low operating temperatures resultsin improved yields of gasoline range hydrocarbons as well as longerperiods before regeneration of the hydrocracking catalyst is required.

According to the method of the present invention, we hsve discovered animprovement in the operation of a hydrotreating reaction for theconversion of nitrogen and sulfur compounds contained in a petroleumfraction into ammonia and hydrogen sulfide, wherein such petroleumfractions comprise hydrocarbons boiling in the range of about 400 1,000F. The improvement of our invention comprises operating a hydrotreatingreaction wherein the hydrogen to hydrocarbon ratio is maintained in therange of 10,000 20,000 SCF/B of hydrocarbon, sufficient to maintain thenitrogen content of the hydrotreated petroleum fraction at one part permillion (ppm) or less. By employing the improvement of the presentinvention, relatively mild hydrotreating operatin conditions oftemperature, pressure, and LHSV may be employed to obtain the desiredlow level of nitrogen compounds in the hydrotreated petroleum fraction.

In another embodiment of our invention, an improved process is providedfor converting a sulfur and nitrogen containing petroleum fractionboiling in the range of 400 l,000 F. into a gasoline fraction boiling inthe range 1 15 400 F. The improved process of the present inventioncomprises hydrotreating a petroleum fraction at a temperature of 600 800F a hydrogen to hydrocarbon ratio of 10,000 20,000 SCF/B, a pressure of1,200 1,500 psig, and a liquid hourly space velocity of O.5l0. Vo/Hr/Vc;separating the hydrotreater reaction effluent into a gas fractioncomprising hydrogen, hydrogen sulfide, and ammonia and a liquidhydrocarbon fraction containing less than 1 ppm nitrogen compounds;scrubbing hydrogen sulfide and ammonia from the hydrotreated gasfraction; charging the scrubbed gas fraction and the hydrotreated liquidfraction to a hydrocracking reaction to convert from about 30 percent toabout percent of the hydrotreated liquid fraction into hydrocarbonsboiling below 400 F.; separating the hydrocracking reaction effluentinto a gas fraction and a liquid fraction; circulating the hydrocrackedgas fraction to the hydrotreating reaction; separating the hydrocrackedliquid fraction into a gasoline fraction and a fraction boiling above400 F recycling the 400 F fraction to the hydrocracking reaction forfurther conversion; and recovering the gasoline fraction as product.

By following the improved method of this invention, the period ofeffective catalyst life for a hydrotreating catalyst is extended byemploying relatively low operating temperatures and the period ofeffective catalyst life for a hydrocracking catalyst is extended bymaintaining the nitrogen content of the hydrocracking reaction charge ata concentration less than 1 ppm. Additionally, only one hydrogen recyclegas system is utilizedfor both the hydrotreating reaction and thehydrocracking reaction. By operating the hydrotreating reaction at apressure somewhat lower than the hydrocrack' ing reaction, it isnecessary to employ only one compressor in the hydrogen recycle gassystem.

BRIEF DESCRIPTION OF THE DRAWING The attached drawing is a schematicrepresentation of a process embodying the improvement of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION The charge stocks which may betreated according to the method of the present invention include ingeneral any mineral oil fraction having an initial boiling point abovethe conventional gasoline range, that is above about 400 F. and havingan end boiling point of up to about 1,000 F. This includes straight rungas oils, coker distillate gas oils, deasphalted crude oils, catalyticor thermal cracked cycle gas oils and the like. These fractions may bederived from petroleum crude oils, shale oils, tar sand oils, coalhydrogenation productsv and the like. Such oils may contain up to about5 weight percent sulfur and up to about 2 weight percent nitrogen.

The hydrotreating reaction contemplated herein is employed to reduce thenitrogen and sulfur content of such charge stocks to low levels, that isto reduce the nitrogen content to 1 ppm or less and the sulfur contentby a concommitant amount. Operating conditions include temperatures inthe range of 600- 800 F pressure of 1,200 1,500 psig, Ll-ISV of 0.5 toVo/l-lr/Vc and hydrogen to hydrocarbon ratios in the range of 10,00020,000 SCF/B, such reaction taking place in the presence of ahydrotreating catalyst.

It is contemplated that temperatures in the lower range will be employedat the beginning of a hydrotreating run when the catalyst is fresh andmore active and that the temperatures will be incrementally increased upto the maximum as the catalyst ages and activity declines. Attempertures below about 600 F. the rate of conversion of sulfur andnitrogen compounds proceeds too slowly. At temperatures above about 800F. the rate of coke deposition upon the hydrotreating catalyst increasesrapidly, thereby decreasing the activity of the hydrotreating catalyst.

Pressures in the range of 1,200 1,500 psig are preferred for the processof the present invention. At pressures below about 1,200 psig thevolumeof gas passing through the hydrotreating reaction zone increasessubstantially thereby increasing the linear velocity to a very highvalue which increases the pressure drop through the reaction zone andwhich may interfere with the proper contact of the hydrocarbon with thecatalyst. Additionally, at low pressures the rate of coke depositionupon the hydrotreating catalyst increases, thereby shortening the periodof effective catalyst life. At pressures above about 1,500 psig thehydrocarbon charge stock may not be sufficiently vaporized to obtain theadvantages of the present invention.

Liquid hourlyspace velocities are-selected to give the desired degree ofsulfur and nitrogen conversion in the ,V hydrotreating reaction zone. Atan Ll-ISV below about pounds. Preferably, an Ll-ISV of from about 0.5 to3 Vo/Hr/Vc is selected.

We have discovered that in hydrotreating a charge stock boiling in therange of 400 1,000 E, the removal of nitrogen and sulfur may besubstantially improved by increasing the hydrogen to hydrocarbon ratiowhile maintaining other operating conditions at set values. We havepostulated that by increasing the hydrogen to hydrocarbon ratio thepartial pressure of the hydrocarbon is decreased and a substantiallylarger portion of such hydrocarbon is allowed to vaporize. When treatinghydrocarbons boiling in the range 400 1,000 F under hydrotreatingconditions, a reaction mixture of hydrogen and hydrocarbon comprises avapor-liquid mixed phase. We postulate that the liquid hydrocarbon formsa film on the surface and in the pores of the catalyst and that vaporreactants must diffuse across such film to reach the active catalystsites. Such diffusion may be relatively slow and may be the ratedetermining step in the hydrotreating reaction. By increasing thehydrogen to hydrocarbon ratio, the partial pressure of the hydrocarbonis decreased which allows additional amounts of hydrocarbon to vaporize.

' Such vaporization reduces the liquid film thickness on sufficient timeto convert the sulfur-and nitrogen comthe hydrotreating catalyst.Additionally, by increasing the amount of hydrogen present in thehydrotreating reaction, the linear velocity of reactants through thereaction zone are substantially increased, which also tends to reducethe film thickness upon the hydrotreating catalyst. It has been foundthat increasing the temperature to increase vaporization of thehydrocarbon is not sufficient to obtain the desired improvement in thehydrotreating reaction. An increase in temperature, without increasingthe hydrogen to hydrocarbon ratio at the same pressure, decreases thehydrogen partial pressure. Consequently, the rate of coke depositionincreases as the temperature is increased, which shortens the period ofeffective catalyst life. We have found that by increasing the hydrogento hydrocarbon ratio the hydrogen partial pressure may be maintained andan increased amount of hydrocarbon may be vaporized at the sametemperature and pressure. To obtain the desired increase inhydrotreating reaction efficiency for conversion of sulfur and nitrogencompounds we have found that hydrogen to hydrocarbon ratios above about10,000 SCF/B must be utilized. Hydrogen to hydrocarbon ratios aboveabout 20,000 SCF/B do not offer a substantial advantage. According tothe method of the present invention, the improvement in the efficiencyof the hydrotreating reaction continues only until all the hydrocarbonis vaporized. Once all the hydrocarbon is vaporized, no furtheradvantage may be obtained by adding increased amounts of hydrogen.

Catalyst which may be employed in the hydrotreating reaction compriseany suitable hydrotreating catalyst. Examples of such hydrotreatingcatalysts include metals of group V! B, metals of group VIII, theiroxides, their sulfides, and combinations thereof. Preferably suchhydrotreating catalysts are supported upon a refractory oxide base suchas alumina, silica, titania, and the like.

The hydrocracking reaction within the contemplation of the presentinvention is for the conversion of hydrocarbons boiling in the range of400 l,000 F. into hydrocarbons boiling below 400 F. We have found thatmaintaining the nitrogen content of a hydrocarbon charged'to'thehydrocracking reaction at 1 ppm or less substantially increases the timeperiod for which the hydrocracking reaction catalyst may be efficientlyutilized. Operating conditions for the hydrocracking reaction comprisetemperatures in the range of 400 800 F., pressures in the range of 5003,000 psig, LHSV in the rangee of 0.5 l Vo/Hr/Vc, hydrogen tohydrocarbon ratios in the range of 3,000 to 15,000 SCF/B,

. such reaction taking place in the presence of a hydrocracking catalystto obtain per pass conversions of hydrocarbons boiling above 400 F. tohydrocarbons boiling below 400 F. of from about 30 percent to about 75percent. It is known that nitrogen compounds decrease the activity ofhydrocracking catalysts thereby requiring increasing temperatures tomaintain the desired conversion of hydrocarbon charge. Such increasingtemperatures increase the rate at which coke is deposited upon thehydrocracking catalyst, thereby shortening the period for which suchhydrocracking catalyst is effective to convert the hydrocarbon into thedesired products.

Temperatures in the range of 400 800 F. are effective to hydrocrackthese selected hydrocarbon charge stocks into the desired gasolineproducts. It is contemplated that temperatures in the low range will beutilized when the hydrocracking catalyst is fresh and has a highactivity and that such temperature will be incrementally increasedduring a hydrocracking run to maintain the desired degree of conversion.Temperatures below about 400 F. do not allow the hydrocracking reactionto proceed at a sufficient rate. Temperatures above about 800 F. causesubstantial cracking of hydrocarbons into very low molecular weighthydrocarbons thereby decreasing the yield of desired gasoline rangehydrocarbons.

The liquid hourly space velocity is selected to provide the desireddegree of hydrocracking. At LHSV below about 0.5 a large volume ofcatalyst is required to treat a selected volume of hydrocarbon. At LHSVabove about the hydrocarbon is not in contact with the hydrocrackingcatalyst for sufficient time to obtain a practical degree of conversion.Preferably, the LHSV is maintained from about 1 to 5 volumes of oil pervolume of catalyst per hour and the operating temperature is adjusted toprovide the desired conversion of hydrocarbon.

In the hydrocracking reaction, the hydrocarbon charge stock is notcompletely converted into desired products boiling below 400 F. in onepass through the hydrocracking zone. Preferably, about 30 percent toabout 75 percent by volume of the hydrocarbon charged to thehydrocracking reaction is converted per pass through the hydrocrackingzone. Unconverted hydrocarbon is recovered from the hydrocrackingreaction effluent and is recycled to the reaction zone. At conversionsbelow about 30 percent, the rate of hydrocarbon recycle is undesirablylarge. At conversions above about 75 percent, the severity of thehydrocracking reaction is such that a considerable proportion of thehydrocarbon is converted into hydrocarbons boiling lighter than thegasoline range. As gasoline is the desired product, conversion ofhydrocarbon into such low boiling hydrocarbons is undesirable.

Hydrogen to hydrocarbon ratios in the hydrocracking reaction aremaintained in the range of from about 3,000 15,000 SCF/B such that therecycle gas separated from the hydrocracking reaction effluent issufficient to provide the desired hydrogen to hydrocarbon ratio of10,000 20,000 SCF/B in the hydrotreating reaction.

Suitable catalysts for use in the hydrocracking reaction comprise thehydrogenation component and a cracking component. Preferably, thehydrogenation component is supported upon a refractory cracking base.For example, suitable cracking bases include mixtures of two morerefractory oxides such as silicaalumina, silica-magnesia,silica-zirconia, aluminaboria, silica-titania, silica-zirconia-titania,acid treated clays, and the like. The preferred cracking bases comprisecomposites of silica and alumina containing about 50 percent silica.Additionally, partially dehydrated zeolitic crystalline molecular sievesof the X or Y crystals types, having relatively uniform pore diametersof about 8 to 14 angstroms and comprising silica, alumina, and one ormore exchangeable zeolitic cations may also be employed as suitablecracking bases. It is preferred to employ molecular sieves having arelatively high SiO /AL O ratio of about 2.5 to 6.0. The most activeforms of molecular sieves are those wherein the exchangeable zeoliticcations are hydrogen and/or divalent metals such as magnesium, calcium,or zinc. The cracking base may comprise amorphous metal oxides,molecular sieves or mixtures thereof. The hydrogenation components arepresent upon the cracking base in an amount from about 0.5 to 25percent. Suitable hydrogenation components are selected from Group VI Bmetals, Group VIII metals, their oxides, their sulfides, or mixturesthereof. Particularly useful hydrogenation components comprise theoxides or sulfides of chromium, tungsten, cobalt, nickel, or thecorresponding free metals or any combination thereof. Alternatively,small proportions, between about 0.5 and 2 percent, of the metalsplatinum, palladium, rhodium or iridium may be employed. The oxides andsulfides of other transition metals may also be used but to lessadvantage than the foregoing.

According to the present invention, the desired high hydrogen tohydrocarbon ratio in the range of l0,000 20,000 SCF/B is maintained inthe hydrotreating zone by circulating a hydrogen containing gasrecovered from the hydrocracking reaction effluent to said hydrotreatingzone. In order to allow the hydrogen containing gas to pass from thehydrocracking zone to the hydrotreating zone without an intermediatecompression step, it is convenient to operate the hydrocracking zone ata higher pressure than the hydrotreating zone. A hydrogen containing gasis recovered from the hydrotreating zone effluent. For the conservationof hydrogen, it is desirable to recycle this gas within the process.However, the hydrogen containing gas recovered from the hydrotreatingefi'luent contains a substantial amount of hydrogen sulfide and ammoniasnd is at a lower pressure than the operating pressure of the hydrocracking zone. Hydrogen sulfide and ammonia have a deleterious effectupon hydrocracking catalyst, and,

in any event, must be removed to maintain the hydrogen concentration inthe recirculating gas stream in the range of from about 60 percent toabout percent which is required to maintain the desired hydrogen partialpressure in the reaction zones. If a hydrocracking catalyst is employedwhich has a high tolerance for hydrogen sulfide and ammonia it ispossible to control the hydrogen sulfide and ammonia concentration inthe recirculating gas stream at a desired value by venting a portion ofthat gas stream. However, since the major component of the recirculatinggas stream is hydrogen, venting wastes a substantial amount of hydrogen.Therefore, it is preferable to treat the hydrogen containing gas streamrecovered from the hydrotreating reaction effluent to remove thehydrogen sulfide and ammonia therefrom. The hydrogen sulfide may beremoved by adsorption into an amine solution, reaction with a causticsolution, or by various solid'adsorbents such as molecular sieves oractivated charcoal. Ammonia may be removed from the gas stream byadsorption into water or upon various solid adsorbents such as molecularsieves or activated charcoal. The hydrogen containing gas stream treatedfor the removal of hydrogen sulfide and ammonia may containnon-condensable light hydrocarbons which tend to increase inconcentration as the process continues. Therefore it may be necessary tovent a small portion of the treated gas stream to maintain theconcentration of such light hydrocarbons at a desired low value.

Hydrogen is consumed in both the hydrotreating reaction and thehydrocracking reaction. Therefore, in a continuous process it isnecessary to add makeup hydrogen to the recirculating gas stream toreplace that consumed in the reactions and that vented from the system.It is not necessary to employ 100 percent pure hydrogen as makeuphydrogen and it is convenient to employ hydrogen streams commonlyavailable in a refinery. Such makeup hydrogen streams should have apurity in the range of 70 to 100 percent hydrogen such that the hydrogenconcentration in the recycle gas stream may be maintained at 60 to 100percent. Since the gas stream recovered from the hydrotreating reactioneffluent is at a lower pressure than the operating pressure of thehydrocracking reaction, it is necessary to provide a compression step totransfer the recirculating gas stream from the hydrotreating reactioneffluent to the inlet of the hydrocracking reaction.

The liquid hydrocarbon component of the hydrotreating reaction effluentafter separation from the gaseous component may still containappreciable amounts of light hydrocarbons, hydrogen sulfide and ammonia.Such low boiling impurities may be removed from the hydrotreatedhydrocarbon by such means as flashing in a low pressure separator, steamstripping, distillation, or other convenient separation means. Thehydrotreated hydrocarbon liquid, treated to remove low boilingimpurities, is adjusted to the desired reaction inlet temperature of thehydrocracking reaction. The temperature of such hydrocarbons may beadjusted by any convenient heat transfer means such as a tired heater,heat exchanger, etc.

Effluent from the hydrocracking reaction is separated into a hydrogencontaining gaseous component and a liquid hydrocarbon component. Thehydrogen containing gaseous component is recirculated to thehydrotreating reaction as hereinabove described. The hydrocrackedhydrocarbon liquid component is passed to a separation means such as afractional distillation column, wherein it is separated into a lighthydrocarbon fraction, a gasoline fraction, and a heavy hydrocar' bonfraction. The light hydrocarbon fraction and gasoline fraction areyielded as products from the process. The heavy hydrocarbon fraction,boiling above about 400 F. represents the unconverted portion of thehydrocarbon charge to the hydrocracking reaction. This heavy hydrocarbonfraction is re-cycled from the separation step to the hydrocrackingreaction for conversion into gasoline and lighter hydrocarbon. Thegasoline fraction obtained as a product herein is suitable as a chargestock for an octane improving process such as, for example, catalyticreforming.

In order to better describe the present invention reference is now madeto the attached drawing. The drawing is a schematic representation ofpreferred embodiment of the present invention. For the sake of claritymany elements such as pumps, valves, instrumentation, etc. commonlyemployed in a commercial process but unnecessary to describe theinvention herein have been eliminated. Such elements may be convenientlyadded by those skilled in the art. As stated, the attached drawingrepresents a preferred embodiment of the present invention and manymodifications and alternations within the scope of the present inventionwill be obvious to those skilled in the art.

In the description of the drawing which follows, liquid flow rates willbe expressed in barrels per hour (b/h) and gas flow rates will beexpressed in thousands of cubic feet per hour (Mscf/h).

Referring now to the drawing, a delayed coker gas oil having a gravityof 28.8 AlPl, an ASTM distillation range of 338 680 F., containing 2,300ppm total nitrogen, 1.46 weight percent sulfur, 36.2 volume percentaromatics, and 14.5 volume percent olefins is treated according to theprocess of the present invention to yield a gasoline product boiling inthe range of 400 F. which is suitable for charge stock to a catalyticreforming process. Delayed coker gas oil at a rate of about 100 b/h vialine 1 and a recirculating gas stream comprising about 86 percenthydrogen at a rate of about 1,320 Mscf/h via line 2 are mixed in line 3to form a mixture having a hydrogen to hydrocarbon ratio of about 11,400 SCF/B. From line 3 the mixture passes into a hydrotreatingreaction zone 4 at a temperature of about 700 F. wherein the reactionmixture is contacted with a hydrotreating catalyst comprising about 3.2weight percent NiO and about 15.7 weight percent M00 supported upon analumina base at an average reaction temperature of about 725 F., an LHSVof about 0.5 Vo/Hr/Vc, and a pressure of 1,500 psig to convertsubstantially all the nitrogen and sulfur to ammonia and hydrogensulfide, respectively. From the hydrotreating zone 41 a reactioneffluent passes via line 5 to cooler 6 wherein the hydrotreating zoneeffluent is cooled to a temperature of about 100 F. to condensesubstantially all the hydrocarbon components boiling above propane. Fromcooler 6, a cooled effluent comprising a liquid component and a gaseouscomponent passes via line 7 into a first high pressure separator 8wherein the gaseous component is separated from the liquid component.The gaseous component comprising hydrogen, hydrogen sulfide and ammonia,recovered via line 9, is passed into a gas treater 10 wherein thehydrogen sulfide and ammonia are separated from the hydrogen byadsorption into a water phase and a diethylamine phase. From the gastreater 10, a gas stream comprising hydrogen substantially free ofhydrogen sulfide and ammonia at a rate of about 1,224 Mscf/h isrecovered via line 11 and mixed with a makeup hydrogen stream comprisingabout 100 percent hydrogen at a rate of about 250 Mscf/h is recoveredvia line 12. A bleed gas stream 34, is removed at a rate of about 2,400SCF/H to remove light hydrocarbons e.g., (C -C produced in the process.The gas mixture comprising hydrogen at a pressure of about 1,250 psig istransferred via line 13 to compressor 14 wherein such gas mixture iscompressed to a pressure of about 1,750 psig. From a compressor 14 thecompressed gas mixture is recovered via line 15 for recirculation to ahydrocracking reaction as will hereinafter be described.

From the first high pressure separator 8 the liquid component of thehydrotreating reaction effluent is recovered via line 16 and passed to alow pressure separator 17 wherein at a pressure of about 250 psig lowboiling components such as hydrogen sulfide, ammonia, some propane andsome lighter hydrocarbons are separated from higher boiling componentsby flashing. The low boiling components comprising hydrogen'sulfide,ammonia, and low boiling hydrocarbons are removed from the low pressureseparator 17 via line 18. Liquid hydrocarbon component from the lowpressure separator, containing less than 1 ppm nitrogen is recovered vialine 19 and is mixed, with a recycle hydrocarbon component in line 21.The hydrocarbon mixture at a rate of about 212 b/h in line 21 andcompressed hydrogen containing gas at a rate of about 1,450 Mscf/h inline 15 are mixed in line 22 and passed into heater 23 wherein themixture is heated to a temperature of about 635 F., at a pressure ofabout 1,850 psig. From heater 23 the heated mixture passes via line 24into hydrocracking zone 25 wherein the mixture is reacted at atemperature of about 650 F., a pressure of about 1,800 psig, a LHSV ofabout 1.0 Vo/Hr/Vc at a hydrogen to hydrocarbon ratio of about 6,000SCF/B in the presence of a sulfided hydrocracking catalyst comprising(before sulfiding) 6 weight percent nickel and 19 weight percenttungsten on a silica-alumina base to convert about 50 percent of thehydrocarbon liquids into hydrocarbons boiling below 400 F. Ahydrocracking reaction effluent, recovered via line 26, passes intocooler 32 wherein normally liquid hydrocarbons are condensed. Coolereffluent passes via line 33 into a second high pressure separator 27wherein the cooler effluent is separated into a gas phase and a liquidphase. The second high pressure separator gas phase, comprising hydrogenis recovered via line 22 and passes into line 3 for mixture withadditional amounts of delayed coker gas oil as hereinabove described.

The second high pressure liquid phase comprising converted andunconverted hydrocarbons is recovered via line 28 and passes intofractionator 29. In fractionator 29 the liquid phase is separated into alight hydrocarbon component, a gasoline product component, and a recyclehydrocarbon component. The light hydrocar bon component comprisinghydrocarbons boiling below about 1 15 F. is recovered from fractionator29 at a rate of about 24 b/h via line 30. The gasoline component boilingin the range of about 1 15 to 400 F. is recovered from fractionator 29at a rate of about 98 b/h via line 31. The recycle hydrocarbon componentcomprising hydrocarbons boiling above 400 F. is recovered fromfractionator 29 at a rate of about 106 b/h via line and is returned vialine 21 for mixture with additional amounts of low pressure separatorhydrocarbon as hereinabove described.

By following the process of the present invention, wherein sulfur andnitrogen containing charge stock is treated at a temperature of about600 F. to about 800 F., a pressure of about 1,200 1,500 psig withhydrogen at a hydrogen to hydrocarbon ratio of 10,000 20,000 SCF/B inthe presence'of a hydrotreating catalyst, a hydrocracking charge stockmay be obtained.

The hydrocracking reaction charge stock obtained by following the methodof this invention contains less than about 1 ppm of nitrogen. Byfollowing the method of the present invention the rate of deactivationof both the hydrotreating catalyst and the hydrocracking catalyst issubstantially diminished thereby allowing sustained periods ofcontinuous operation of long duration in the range of about 12 to 18months before the process must be discontinued to allow regeneration ofthe hydrotreating and hydrocracking catalyst.

By operating the hydrotreating reaction zone at pressures lower than thehydrocracking reaction zone an additional advantage is obtained in thathydrogen containing gas recovered from the second high pressureseparator 27 may be circulated directly to the hydrotreating zone 4without an intermediate compression step, and the hydrogen containinggas recovered from the effluent of the hydrotreating zone 4 may becompressed and returned to hydrocracking zone 25 thereby conservinghydrogen. Only one compression step is required to circulate thehydrogen containing gas through both the hydrocracking zone 25 and thehydrotreating zone 4.

Results analogous to those described in the foregoing description of thedrawing are obtained when other hydrocracking catalysts, and operatingconditions, other feed stocks, and other hydrotreating catalysts andconditions within the broad purview of the above disclosure areemployed. Also modifications and variations of this above describedprocess will occur to those who are skilled in the art. Therefore it isnot intended to limit the invention to the details of the specificembodiment described but only to the limitations contained within thespirit and scope of the appended claims.

We claim:

1. In a method for converting a sulfur and nitrogen containinghydrocarbon charge stock boiling in the range of from about 400 F. toabout 1,000 F. into a gasoline product boiling in the range of about F.to about 400 F., wherein the hydrocarbon charge stock is treated withmolecular hydrogen in the presence of a hydrotreating catalyst toconvert sulfur compounds into hydrogen sulfide and nitrogen compoundsinto ammonia, wherein effluent from the hydrotreating reaction zone isseparated into a first hydrocarbon liquid phase and a gas phasecomprising hydrogen, hydrogen sulfide, and ammonia, wherein thehydrotreated liquid phase is contacted with molecular hydrogen in thepresence of a hydrocracking catalyst to convert hydrocarbons boilingabove about 400 F. into hydrocarbons boiling below about 400 F., whereinefiluent from the hydrocracking reaction is separated into a gaseousphase comprising hydrogen and a second liquid hydrocarbon phase, whereinthe second liquid hydrocarbon phase is separated into a fraction boilingbelow about 400 F. and a fraction boiling above about 400 F. and whereinthe hydrocracked fraction boiling above about 400 F. is recycled to thehydrocracking reaction for conversion into hydrocarbons boiling below400 F.; the improvement which comprises:

a. Hydrotreating the hydrocarbon charge at a tem perature of from about600 F. to about 800 F a pressure of from about 1,200 to about 1,500psig, a liquid hourly space velocity of from about 0.5 to about 10volumes of oil per volume of catalyst per hour, and with a hydrogen tohydrocarbon ratio of from about 10,000 to about 20,000 SCF/B;

b. separating the hydrotreating reaction effluent into a hydrotreatedgas phase comprising hydrogen, hydrogen sulfide, and ammonia and a firstliquid hydrocarbon phase containing about 1 ppm or less nitrogen;

c. treating gas phase from the hydrotreating step for removal ofhydrogen sulfide and ammonia therefrom;

d. compressing treated gas of step (b), comprising hydrogen, to apressure in the range of about 1,400 psig to about 2,000 psig, above thepressure of hydrotreating step (a);

e. hydrocracking the hydrotreated liquid phase of step (b) in thepresence of the compressed treated gas of step (c) at a pressure in therange of about 14 1,400 psig to about 2,000 psig; f. separating thehydrocracking reaction effluent into a hydrocracked liquid phase and agas phase comprising hydrogen; and g. circulating the gas phase of step(f) for contact with additional hydrocarbon charge in step (a). 2. Themethod of claim 1 wherein the hydrocracking reaction is performed at atemperature in the range of about 400 F. to about 800 F., at a liquidhourly space velocity of from about 0.5 to about 15 vol. oil/hr./vol.catalyst, and wherein the hydrogen to hydrocarbon ratio is in the rangeof from about 3,000 to about 15,000 SCF/B.

2. The method of claim 1 wherein the hydrocracking reaction is performedat a temperature in the range of about 400* F. to about 800* F., at aliquid hourly space velocity of from about 0.5 to about 15 vol.oil/hr./vol. catalyst, and wherein the hydrogen to hydrocarbon ratio isin the range of from about 3,000 to about 15,000 SCF/B.